Handbook of Electrical Engineering

228 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 9.5 Cut-off and prospective current of fuses.

For the selection of fuses the prospective symmetrical rms value of the off-set fault current
Ifrmsis calculated as,

Ifrms=

Ifpka

2

√

2

=

147 , 654

2

√

2

= 52 ,204 amps

From the graphical characteristics given by the manufacturer the maximum fuse rating can be
selected, see Figure 9.5.

The maximum fuse rating is 315 A and so the choice of 200 A is satisfactory.
Select a cable and check that its I-squared-t characteristics are adequate.
From Table 9.17 the derating for ambient temperatureKairis 0.92 and assume no derating for
the grouping of cables. Hence the nearest cable rating to carry 200 amps at 35◦Chasa70mm^2 cross-
sectional area. This cable has a thermal energy constant K of 143. Hence its time-current characteristic
is given by two or more points on a straight line on a log-log graph. From the following,

√

t=

KA

I

=

143 × 70

I

=

10010. 0

I

Several suitable points on the graph are given in Table 9.27.